The present invention relates to an apparatus for controlling an output of a laser diode over temperature so as to control and optimally maintain constant its extinction ratio, and a method for initially setting HIGH and LOW outputs of the laser diode so as to have a desired extinction ratio.
Various control circuits have been proposed in the prior art for maintaining an output of a laser diode. FIG. 1 illustrates various characteristic curves for a laser diode whereby current I versus power P is graphed, the three curves illustrating data for a laser diode at three distinct temperatures. As is evident and known, a pedestal bias current 4 of the laser diode increases with temperature. One method of controlling an output of a laser diode is to incorporate a thermoelectric cooler into a laser package so as to keep the diode at a constant temperature so that its pedestal bias current remains constant. Accordingly, an extinction ratio of the laser diode, defined as the ratio between its HIGH power output divided by its LOW power output can easily be maintained constant. A disadvantage of such proposals is that thermoelectric cooler designs tend to increase the laser cost, and decreases reliability of the laser diode since any failure in the thermoelectric cooler device or its circuitry will result in the applied bias current being inappropriate as the temperature of the laser diode varies.
Circuits have also been proposed whereby a LOW pedestal power is monitored successively over time for the laser diode and incrementally increased or decreased in response to comparison to a preset dynamic threshold. One such circuit is described by Geller, U.S. Pat. No. 5,036,189, assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference. Though the Geller circuit readily establishes and maintains an appropriate LOW pedestal bias current, it does not control the HIGH power output of the laser. Referring back to FIG. 1, since a slope of the current versus power output of a laser changes over temperature, for a constant modulation current I.sub.M, the laser extinction ratio (the ratio between the laser HIGH power divided by the laser LOW power) will change over time. If the extinction ratio becomes too small, increased noise results at an optical receiver detecting an output of the laser which results in undesirable bit errors.
It has also been proposed to utilize circuitry which maintains the average laser modulation power constant over time. Such circuits also suffer from a disadvantage that the laser output extinction ratio will change with temperature. Specifically, referring to FIG. 1, curve 3 illustrates how such circuitry works on an output of the laser and illustrates that noise detected by the receiver when the laser emits a LOW output is undesirably high, i.e. greater than the power output at the laser threshold pedestal.